EXAFS ANALYSES OF MANGANESE ENZYMES James E. Penner-Hahn¹, Timothy L. Stemmler¹, Pamela J. Riggs-Gelasco¹, and Charles F. Yocum², Departments of Chemistry¹ and Biology², The University of Michigan, Ann Arbor, MI 48109-1055 USA

X-ray absorption spectroscopy has been used to characterize the local Mn environments of the multinuclear Mn clusters in Mn catalase and in the photosynthetic oxygen evolving complex (OEC). Manganese catalase contains a dinuclear Mn site, while the OEC contains a tetranuclear Mn cluster, together with Ca and Cl as inorganic co-factors. Although the Mn ions are magnetically interacting in reduced, Mn(II)/Mn(II), catalase, the enzyme lacks a detectable Mn***Mn EXAFS signal. This is similar to findings for other dinuclear Mn enzymes, but distinct from the behavior of crystallographically characterized Mn models. In contrast, the OEC contains readily detectable Mn***Mn interactions at 2.7 and 3.3 Å. The dependence of the OEC structure on chemical perturbations points to the presence of two functionally distinct Mn dimers. Large, hydrophobic reductants such as p-dihydroquinone, completely disrupt the cluster structure, giving a state containing two Mn(II) ions. Small, hydrophillic reductants such as NH₂OH give a reduced enzyme with a structure that is essentially unchanged from the resting state. One of the striking differences between Mn catalase and the OEC is the oxidation state of the resting enyzmes, with Mn(II) favored in the former, and Mn(III)/Mn(IV) favored in the latter. Detailed characterization of their nearest neighbor environments, and comparisons of these with crystallographically characterized Mn models suggests that the stability of lower oxidation states in the catalase is a consequence of the presence of nitrogen containing ligands as compared to alkoxide/phenoxide/or hydroxide ligands in the latter.